1. Field of the Invention
The present invention relates to an image display method and an image display apparatus for displaying an image on image display means such as a CRT display or a liquid crystal display device. More specifically, the present invention relates to an image display method and an image display apparatus for displaying an image on image display means after carrying out gradation conversion processing according to a predetermined gradation conversion table on an image signal.
2. Description of the Related Art
When a visible image is displayed on image display means such as a CRT display or a liquid crystal display device based on an image signal, gradation correction according to a gradation conversion table is carried out in advance on the image signal.
This is because a gradation characteristic (a characteristic of output brightness to an input signal [image signal]) of image display means shows a nonlinear characteristic as shown in FIG. 5B while a displayed image is assumed to have a linear gradation characteristic as shown in FIG. 5C, for example. Therefore, preset gradation conversion processing according to a gradation conversion table as shown in FIG. 5A is carried out on an input signal Din to obtain an output signal Dout, for example. By inputting the output signal Dout to image display means having a gradation characteristic shown in FIG. 5B, a relationship (gradation characteristic) between the original input signal Din and output brightness N can become linear [FIG. 5C].
However, a display screen (a fluorescent screen of a CRT display, for example) of image display means or an electron gun thereof (backlight of a liquid crystal display device) for emission display degrades with time, and brightness of the screen changes in some cases.
In other words, since the gradation characteristic of the image display means initially shows brightness having a gradation in a high signal range [a curve (i) in FIG. 2A], the gradation characteristic of the input signal Din after the gradation correction shows linearity over an entire dynamic range (0xcx9c100%) of the input signal Din as shown by a curve (i) in FIG. 2B. However, if the brightness degradation sequentially progresses from a curve (ii) to (iii) or (iv) in FIG. 2A, the gradation becomes gradually flat from the high signal range and the gradation characteristic of the input signal Din after the gradation correction cannot display a gradation in the high signal range of the input signal Din, as shown by curves (ii), (iii), and (iv) in FIG. 2B.
For example, if the degradation is as shown in FIG. 2B(ii), a 90% value and a 100% value of the input signal Din show brightness N0, which means that gradation is not represented in this range. If the input signal Din is a signal of a medical radiation image (a negative image), a gradation in the high density range cannot be represented. Therefore, if an area of interest in the image exists in this high density range, observation and reading of the image is hindered and it becomes difficult to carry out a useful diagnosis.
There has been proposed a technique for preventing degradation of a final gradation characteristic regardless of brightness degradation (see Japanese Unexamined Patent Publication No. 9(1997)-212144), by correcting a gradation conversion table in accordance with brightness degradation of image display means (including a case of having gradation conversion tables corrected at a plurality of levels in advance).
According to this technique, as shown by curves (i), (ii), (iii), and (iv) in FIG. 3, gradation conversion tables for correcting degradation of gradation characteristics due to brightness degradation are used for each predetermined level of brightness degradation [(i), (ii), (iii), and (iv) in FIG. 2A]. By selecting and using one of the gradation conversion tables corresponding to detected brightness degradation, linearity of the gradation characteristic of the brightness N to the value of the input signal Din after the correction can be maintained over almost all the dynamic range of the input signal Din, as shown by curves (i), (ii), (iii), and (iv) in FIG. 4.
Brightness degradation is detected by using an image quality SMPTE pattern (a test pattern for image quality control recommended by the Society of Motion Picture and Television Engineers [in USA]; see FIG. 8). An observer views a contrast pattern P included in the SMPTE pattern and detects whether or not a gradation in a high signal range (a gradation between a brightness N0 corresponding to a maximum signal value (100%) and a brightness N1 corresponding to a signal value which is 95% of the maximum signal value) is represented. In this manner, the gradation detection is easily carried out.
In the contrast pattern P in the SMPTE pattern shown in FIG. 8, a small square is formed at the center of a large square and the brightness N1 corresponding to the signal value which is 95% of the maximum signal value (brightness after the gradation conversion using the gradation conversion table at the initial level shown in FIG. 3(i)) is displayed in the inner square while the brightness N0 corresponding to the maximum signal value (brightness after the gradation conversion using the gradation conversion table at the initial level) is displayed in an area of the outer square excluding the inner square. Before brightness degradation occurs, a brightness difference between the inner square and the outer area thereof can be recognized. However, after brightness degradation has occurred, no brightness difference is recognized between the inner square and the outer area thereof, since the gradation in the high signal range, that is, in a low brightness range has become flat. Therefore, gradation characteristic degradation can be detected by the change in the gradation recognition performance.
In the SMPTE pattern, a contrast pattern using brightness between 0% and 5% signal values is also available in a low signal range, that is, in a high brightness range. However, in the low signal range, the brightness degradation described above is not directly connected to the gradation degradation. Therefore, the gradation recognition performance does not change.
In the case where a contrast pattern shown in FIG. 9A (a contrast pattern using the brightness between 100% and 90% signal values) is used as the contrast pattern in the above-described high signal range where the gradation recognition performance changes, the brightness/input signal characteristic after gradation correction according to the original gradation conversion table [FIG. 3(i)] becomes the curve in FIG. 2B(ii) when the brightness of the image display means has degraded as shown in FIG. 2A(ii). In this case, the contrast pattern becomes as shown in FIG. 9B. As a result, the observer cannot recognize the gradation in the contrast pattern. Therefore, the gradation degradation is detected and the gradation conversion table is changed to a second gradation conversion table shown in FIG. 3(ii). The final brightness/input signal characteristic after the change becomes as shown in FIG. 4(ii), and the contrast pattern is changed to a gradation-recognizable pattern shown in FIG. 9C.
Gradation degradation gradually progresses from FIG. 2A(i) to (ii), then to (iii) and further to (iv). However, since the contrast pattern is not necessarily displayed each time an image is displayed on image display means, brightness degradation has progressed in some cases from the state (i) where no degradation is observed to (iii) or (iv) when it is realized. In this case, the gradation recognition performance of the contrast pattern is the same as the performance [shown by FIG. 9B] in the brightness degradation in the state shown in FIG. 2A(ii), even if the brightness degradation is actually in the state of (iii) or (iv). Therefore, the observer does not detect a degree of degradation by using the contrast pattern alone although the observer detects the degradation itself. As a result, the gradation conversion table enabling optimal gradation correction cannot be selected from the plurality of gradation conversion tables prepared in advance according to the brightness degradation. Therefore, it is necessary to find a gradation conversion table enabling the contrast pattern shown in FIG. 9B to have an adequate gradation characteristic shown in FIG. 9C by using each of the gradation conversion tables shown in FIG. 3(ii), (iii) and (iv) in trial and error, which imposes a troublesome operation on the observer.
The present invention has been conceived based on consideration of the above problems. An object of the present invention is therefore to provide an image display method and an image display apparatus for enabling easy selection of a gradation conversion table to be adopted in accordance with a degree of brightness degradation of image display means.
An image display method and an image display apparatus of the present invention have in advance a plurality of gradation conversion tables for correcting a gradation characteristic which degrades in accordance with brightness degradation of image display means, and displays contrast patterns at a plurality of levels changing gradation recognition performance in accordance with a level (degree) of the brightness degradation in order to detect the level of the brightness degradation at a glance.
In other words, the image display method of the present invention is a method of displaying on image display means a visible image represented by an image signal having been subjected to gradation conversion processing according to a predetermined gradation conversion table. The image display method comprises the steps of:
preparing a plurality of the gradation conversion tables according to a plurality of levels of brightness degradation of the image display means in order to correct a gradation characteristic degrading in accordance with the brightness degradation;
displaying contrast patterns at a plurality of levels having gradation recognition performance changing in accordance with the plurality of the levels of the brightness degradation of the image display means;
selecting one of the gradation conversion tables adequate for correcting the gradation characteristic in accordance with a change of the gradation recognition performance of the contrast patterns at the plurality of the levels; and
carrying out the gradation conversion processing according to the gradation conversion table having been selected.
The number of gradation conversion tables prepared according to the levels of the brightness degradation is a number corresponding to the levels of brightness degradation to be corrected. Therefore, if the levels of brightness degradation to be corrected are 2, 2 gradation conversion tables are sufficient. Since it is preferable for the levels of brightness degradation to be 3 or more, the number of the gradation conversion tables is preferably 3 or more.
The number of contrast patterns having changing gradation recognition performance is preferably a number corresponding to the levels of brightness degradation to be corrected, as in the case of the gradation conversion tables. However, displaying the contrast patterns at more levels, that is, displaying more contrast patterns than the number of levels of the brightness degradation to be corrected, is permissible.
xe2x80x9cPreparing a plurality of the gradation conversion tables according to a plurality of levels of brightness degradationxe2x80x9d refers to preparing a plurality of gradation conversion tables having different conversion characteristics. Alternatively, by preparing only one gradation conversion table but having correction tables for correcting output of the gradation conversion table to combine the only gradation conversion table and the correction tables, a plurality of gradation conversion tables may be used in practice.
The image display means includes emission display means such as a CRT display and a liquid crystal display device.
An image display apparatus of the present invention is an apparatus for realizing the above image display method. The image display apparatus displays on image display means a visible image represented by an image signal having been subjected to gradation conversion processing by a gradation correction unit thereof for carrying out the gradation conversion processing on the image signal according to a predetermined gradation conversion table. The image display apparatus comprises:
a plurality of gradation conversion tables for a plurality of levels of brightness degradation of the image display means in order to correct a characteristic of a gradation degrading with the brightness degradation;
a contrast pattern storing unit for storing contrast patterns at a plurality of levels having gradation recognition performance changing in accordance with the levels of brightness degradation of the image display means, and gradation conversion table selection means for selecting one of the gradation conversion tables adequate for correcting the gradation in accordance with a change in the gradation recognition performance of the contrast patterns at the plurality of the levels, and
the gradation correction unit carries out the gradation conversion processing on the image signal according to the gradation conversion table having been selected.
According to the image display method and apparatus of the present invention, by simultaneously displaying on the image display means the contrast patterns at the plurality of levels having the gradation recognition performance changing in accordance with the brightness degradation level of the image display means such as a CRT display or a liquid crystal display, the level (degree) of the brightness degradation can be detected at a glance and the gradation conversion table adequately correcting the gradation at the detected degradation level can be selected easily from the plurality of gradation conversion tables prepared to correct the gradation characteristic degrading in accordance with the brightness degradation of the image display means.